Preorbital gland

The preorbital gland is a paired exocrine gland found in many species of hoofed animals, which is homologous to the lacrimal gland found in humans. These glands are trenchlike slits of dark blue to black, nearly bare skin extending from the medial canthus of each eye. They are lined by a combination of sebaceous and sudoriferous glands, and they produce secretions which contain pheromones and other semiochemical compounds. Ungulates frequently deposit these secretions on twigs and grass as a means of communication with other animals. The preorbital gland is a paired exocrine gland found in many species of hoofed animals, which is homologous to the lacrimal gland found in humans. These glands are trenchlike slits of dark blue to black, nearly bare skin extending from the medial canthus of each eye. They are lined by a combination of sebaceous and sudoriferous glands, and they produce secretions which contain pheromones and other semiochemical compounds. Ungulates frequently deposit these secretions on twigs and grass as a means of communication with other animals. The preorbital gland serves different roles in different species. Pheromone-containing secretions from the preorbital gland may serve to establish an animal's dominance (especially in preparation for breeding), mark its territory, or simply to produce a pleasurable sensation to the animal. Because of its critical role in scent marking, the preorbital gland is usually considered as a type of scent gland. A further function of these glands may be to produce antimicrobial compounds to fight against skin pathogens. Antimicrobial compounds found in these glands may be biosynthesized by the animal itself, or by microorganisms that live in these glands. Deer have seven major external scent glands, distributed throughout their bodies. These glands are the forehead glands (located on the forehead), the preorbital glands (located below the eyes), the nasal glands (located inside the nostrils), the interdigital glands (located between the toes), the preputial gland (located inside the foreskin of the deer's penis), the metatarsal glands (located outside of the hind legs), and the tarsal glands (located inside of the hind legs). Though it is not their primary function, the salivary glands also function as scent glands. Deer rely heavily on these scent glands to communicate with other members of their species, and possibly even with members of other species. A deer may rub its preorbital gland (e.g., on a branch) purely for pleasure. The two major species of deer found in North America are the white-tailed deer (Odocoileus virginianus) and the mule deer (Odocoileus hemionus). The most important sense in these animals is olfaction (the sense of smell)—so much so that they have an accessory olfaction system.The vomeronasal organ, located at the base of the nasal cavity, is the sensory organ for this system. Besides locating food and water, deer rely on their two separate olfactory systems to detect the presence of predators, as well as to supply them with information about the identity, sex, dominance status and reproductive status of other deer. The preorbital gland of O. virginianus is about 22 millimeters (0.87 in) in length, while that of O. hemionus is roughly 40 millimeters (1.6 in) in length. In black-tailed deer (O. h. columbianus), a subspecies of O. hemionus, the preorbital gland measures about 32 millimeters (1.3 in). In all of these animals, the preorbital glands are surrounded by muscle which is under voluntary control, at least to some extent. It is not entirely clear whether the preorbital gland secretions of North American deer emit an odor that is significant in terms of chemical communication. Most of the time, the glands remain in the closed position, but they are capable of opening them in certain circumstances. For example, a rutting male may dilate its preorbital glands in order to signal aggression to another nearby male. Female deer often open their glands while caring for their young. In juvenile red deer (Cervus elaphus), the preorbital gland appears to play a role in the response to stress. The preorbital gland is closed in a relaxed calf, whereas it is opened in a stressed calf. One example of this is the signalling of hunger and satiety. Fawns open their preorbital glands as a signal that they are hungry, and close the gland after feeding, when they are no longer hungry. Other than during the rut (mating season) and for the first six months after giving birth, the adult Indian muntjac (Muntiacus muntjac) is a solitary animal. Adult males in particular are well spaced and marking grass and bushes with secretions from their preorbital glands appears to be involved in the acquisition and maintenance of territory. The bovids (family Bovidae) comprise some 140 species of ruminants in which at least the males bear unbranched, hollow horns covered in a permanent sheath of keratin. Most species of bovids have means of spacing themselves across their habitat; territorial behavior is the most consistent type of spacing behavior.